1. Field of the Invention
This invention relates generally to the recovery of digital data from an analog video signal and, more particularly, to the recovery of digital data from an analog video signal by using a color subearrier in the video signal for clock synchronization in the transmitter and the receiver.
2. Discussion of the Related Art
Various video transmission systems, such as cable television, closed circuit television, etc. utilize a transmitter to transmit an analog video signal, and a receiver to receive the analog video signal and convert it to an appropriate signal for viewing. Most of these transmission systems use data encoding schemes to encode digital data and/or data bit symbols onto the analog video signal for many things such as descrambling authorization, on-screen displays, teletext, auxiliary capabilities, closed caption, etc. When digital data is encoded onto an analog video signal, some procedure must be provided to synchronize the transmitted digital data with a clock signal in the receiver for data recovery. This technique is generally referred to as "bit synchronization," and is typically achieved by phase-locking a local clock source in the receiver with clock phase information transmitted with the digital data.
Typically, the digital data transmission format is of a data burst nature, where there are relatively long intervals of time between sequences of digital data in the analog signal. In this context, data burst refers to sequences or groups of digital data between intervals of no data in the analog signal without data. Color burst is a term of art that refers to the 8 to 10 cycles of sine wave at the beginning of each line of video used to derive color demodulation information for each segment of video information. Generally, the digital data is transmitted in the vertical blanking interval of the video signal. The vertical blanking interval is a segment of the analog video signal at the beginning of each video field that does not include video data. In this format the digital data bursts occur about 16 to 20 milliseconds apart.
For known video signal transmission systems, when digital data is transmitted in a burst format, some of the bits of data in each burst must be sacrificed to provide clock phase information since the receiver's data clock cannot accurately roll through the time intervals where no digital data is being received, and still be synchronized to the data. Because these digital data bits must be sacrificed to provide the clock phase information, these bits are not available to provide the desired information. Therefore, additional data bits are required beyond those bits that include the information. Because the portion of the analog video signal available to transmit digital data is limited, the digital data bits carrying the clock phase information may limit the amount of information that can be transmitted in each data burst.
What is needed is a bit synchronization technique that synchronizes the data bits in a transmitted analog video signal to a clock phase in the receiver so that the data information can be accurately extracted from the video signal, where the technique does not use data bits in the data information to provide clock phase information. It is therefore an object of the present invention to provide such a bit synchronization technique.